1. Field of the Invention:
This invention relates to graft copolymers containing cationic groups in the polymer chain.
2. Description of the Related Art:
It is well known to blend low Tg (glass transition) elastomers with more brittle thermoplastic polymers having a higher Tg to yield blends with improved toughness and impact strength. The desired properties are only obtained by achieving a fine dispersion of the elastomer in the thermoplastic polymer.
It is also well known to form blends of relatively highly unsaturated elastomers such as polybutadiene or natural rubber with elastomers having less or no ethylenic unsaturation, e.g., butyl rubber, halogenated butyl rubber, copolymers of isobutylene and a para-alkylstyrene, ethylene/propylene or ethylene/propylene/diene rubber and the like, in order to enhance the properties of vulcanizates based on such blends. Such property enhancement includes improved oil and grease resistance, improved resistance to oxygen and ozone degradation, improved abrasion resistance and improved impermeability to air, water vapor and many organic solvents.
One of the major problems associated with the formation of such blends is that most of these polymer components are mutually incompatible. Poor mutual adhesion at the blend phase boundaries in such systems results in a diminishment of physical properties of the polymer blend.
It is well established that the use of appropriately structured polymer compatibilizers as additives in such systems can markedly enhance physical properties. These additives are typically block or graft copolymers in which polymer segments interact in a favorable manner with each polymer phase of the blend. This xe2x80x9cbridgingxe2x80x9d at the interfacial blend region enhances the interfacial adhesion, and improves the dispersion and size of the discrete blend phase and improves mechanical properties. Thus, for example, a block or graft copolymer containing both polybutadiene and polystyrene polymer units can serve as a compatibilizer in a polymer system comprising a blend of polystyrene and polybutadiene.
Analogs of halogenated butyl rubber have been recently discovered which comprise the halogenated product of a random copolymer of a C4 to C7 isomonoolefin, e.g. isobutylene, and a para-alkystyrene, e.g. para-methylstyrene, such as disclosed in U.S. Pat. No. 5,162,445. Although these materials are devoid of olefinic backbone unsaturation, the presence of a benzylic halogen, e.g. bromine, permits facile crosslinking or curing of the polymers using, for example, zinc oxide/stearic acid curing systems. In addition, functionalized versions of these copolymers may be prepared by nucleophilic substitution of the benzylic halogen present in the polymer molecules with a reagent reactive therewith to provide polymers containing amine, acid, ester, amide, onium and the like functionality. Grafted polymers may also be prepared by a graft-onto reaction wherein the halogenated copolymer is reacted in solution with a polymeric nucleophile such as polystyryl lithium. These graft-onto reactions lead to the production of solvent soluble grafted copolymers which are disclosed to be useful as compatibilizers for polymer blends, e.g., blends of polyisobutylene, butyl rubber or halogenated copolymers of isobutylene and para-methylstyrene with polystyrene.
The present invention provides graft copolymers which are polymeric onium salts of polymers and copolymers containing halogenated para-alkylstyrene units in the backbone polymer chain.
More specifically, the invention provides a graft copolymer containing cationic units in the polymer chain having the structure: 
wherein R and Rxe2x80x2 are independently selected from the group consisting of hydrogen and; a C1 to C4 alkyl group, Z is selected from the group consisting of nitrogen and phosphorous, R1 and R2 are the same or different alkyl radicals containing 1 to 4 carbon atoms, R3 is a direct link or an organic radical containing 1 to 6 carbon atoms, X is halogen and PL is a polymer chain segment derived from at least one anionically polymerized monomer.
The invention also provides a process for preparing a graft copolymer comprising:
a) polymerizing one or a mixture of anionically polymerizable monomers under anionic polymerization conditions using, as a polymerization initiator, a compound having the formula R1 R2 Z R3xe2x80x94Li to form a polymer having the structure R1 R2 Z R3xe2x80x94PL; and
b) reacting said polymer under onium salt-forming conditions with a copolymer containing polymerized monomer units in the polymer chain having the structure: 
wherein a graft copolymer containing cationic units pendant to the backbone polymer chain and having the structure described above is obtained.
Graft copolymers of this invention are useful as compatibilizers for immiscible polymer blends and are also useful in their own right as thermoplastic elastomers having improved mechanical properties.
The graft copolymers within the scope of this invention differ from the graft copolymers disclosed in U.S. Pat. No. 5,162,445 in their structure and method of preparation. In the ""445 patent, a xe2x80x9clivingxe2x80x9d anionically polymerized polymer or a carboxylate thereof is subjected to a nucleophilic displacement reaction involving benzylic halogen present in a backbone polymer containing polymerized, halogenated para-methylstyrene. In the present invention, graft copolymers are prepared by reacting an anionically polymerized polymer polymerized using a tertiary amino or phosphino lithium initiator. Subsequent reaction of the phosphino or amino group present at the polymer end with a backbone polymer containing the benzylic halogen results in the formation of an ammonium or phosphonium cationic group situated between the backbone chain and the pendant graft polymer. This group enhances both the mechanical properties of the graft copolymer as well as the dispersion therein of a negatively charged filler materials such as carbon black and colloidal silica.
Polymers which may be used as the backbone segment in preparing the graft copolymers of this invention may be thermoplastic or elastomeric polymers. Suitable thermoplastic polymers include halogenated, e.g., brominated, poly (para-methylstyrene) and random copolymers of halogenated para-methylstyrene with one or more other monomers such as non-halogenated para-methylstyrene, styrene, t-butylstyrene, alpha-methylstyrene, acrylonitrile, alkylacrylates or methacrylates, butadiene and the like.
Suitable elastomeric copolymers used as the backbone segment include random halogenated copolymers of less than 50 wt % of a para-alkylstyrene such as para-methylstyrene and at least 50 wt % of one or more elastomer-forming monomers such as C4 to C7 isomonoolefin, butadiene, isoprene and the like.
Preferred copolymers forming the backbone segment of the graft copolymers are the halogenation product of random elastomeric copolymers of a C4 to C7 isomonoolefin, such as isobutylene, and a para-alkyl styrene comonomer, preferably para-methylstyrene containing at least about 80%, more preferably at least about 90% by weight of the para isomer, and wherein at least some of the alkyl substituent groups present in the styrene monomer units contain halogen. Preferred materials may be characterized as isobutylene interpolymers containing the following monomer units randomly spaced along the polymer chain: 
wherein R and Rxe2x80x2 are independently hydrogen, lower alkyl, preferably C1 to C4 alkyl, and X is bromine or chlorine, and wherein the interpolymer is otherwise substantially free of ring halogen or halogen in the polymer backbone chain. Preferably R and Rxe2x80x2 are each hydrogen. Up to about 65 mole % of the para-alkylstyrene present in the interpolymer structure may be the halogenated structure (2) above, more preferably from about 5 to 60 mol %.
Most useful of such materials are elastomeric random copolymers of isobutylene and para-methylstyrene containing from about 0.5 to about 20 wt % para-methylstyrene wherein up to about 60 mole % of the methyl substituent groups present on the benzyl ring contain a bromine or chlorine atom, preferably a bromine atom. These copolymers have a substantially homogeneous compositional distribution such that at least 95% by weight of the polymer has a para-alkylstyrene content within 10% of the average para-alkylstyrene content of the polymer. They are also characterized by a narrow molecular weight distribution (Mw/Mn) of less than about 7, more preferably less than about 5.0, a preferred viscosity average molecular weight in the range of from about 300,000 up to about 2,000,000, and a preferred number average molecular weight in the range of from about 10,000, preferably 25,000, to about 1,000,000, as determined by Gel Permeation Chromatography.
The copolymers may be prepared by slurry polymerization of the monomer mixture using a Lewis Acid catalyst, followed by halogenation, preferably bromination, in solution in the presence of halogen and a radical initiator such as heat and/or light and/or a chemical initiator.
Preferred brominated copolymers generally contain from about 0.01 to about 10 mole % of bromomethyl groups, most of which is monobromomethyl, with less than 0.05 mole % dibromomethyl substituents present in the copolymer. These copolymers, their method of preparation, their method of cure and graft or functionalized polymers derived therefrom are more particularly disclosed in U.S. Pat. No. 5,162,445, the complete disclosure of which is incorporated herein by reference. Graft polymer derivatives from these copolymers are particularly disclosed in Examples 96-106.
The graft polymer (PL) segments of the graft copolymers of this invention are derived from one or a mixture of anionically polymerizable monomers such as butadiene, isoprene, vinyl aromatic monomers such as styrene, t-butyl styrene, alpha-methylstyrene and the like; alkyl acrylates and methacrylates, acrylonitrile and like anionically polymerizable monomers. The graft PL segments may be thermoplastic (Tg above 20xc2x0 C.) or elastomeric (Tg below 20xc2x0 C.) in nature. The graft PL segments may be homopolymer segments or copolymer segments, including random copolymers or block copolymers. The block copolymer segments may comprise diblock (AB) or polyblock (ABA, ABAB, etc.) segments, as well as tapered block segments. The molecular weight of these graft PL segments generally may range from about 1,000 to 100,000 g/mole.
Nucleophilic initiators which may be used in the anionic polymerization of the graft PL segments in accordance with this invention include tertiary amino or phosphino lithium compounds having the structure: R1 R2 Z R3xe2x80x94Li, wherein Z is nitrogen or phosphorous, R1 and R2 are the same or different alkyl radicals containing 1 to 4 carbon atoms and R3 is a direct link or an organic radical containing 1 to 6 carbon atoms, preferably a hydrocarbon radical containing 1 to 4 carbon atoms. The most preferred initiators are tertiary amino lithium compounds having the structure: 
wherein R1 and R2 are alkyl radicals containing 1-3 carbon atoms and R3 is a hydrocarbon group containing 2-4 carbon atoms, and tertiary phosphino lithium compounds having the structure: 
wherein R1 and R2 are alkyl radicals containing 1-3 carbon atoms and R3 is a hydrocarbon group containing 2-4 carbon atoms.
Suitable initiator compounds include 3-dimethylaminopropyl-lithium, 3-diethylaminopropyl-lithium, diethyl amino-lithium, diethylphosphino-lithium, 3-dimethyl-phosphinopropyl-lithium, 3-dimethylaminobutyl-lithium and like compounds.
A particularly preferred initiator is a tertiary amino lithium compound wherein R1 and R2 are each methyl and R3 is propyl, e.g., 3-dimethylaminopropyl-lithium. This initiator, its method of preparation and its use for preparing diolefin and styrene polymers is disclosed in Eisenbach et al, Eur. Polym J. (1975), 11(10), p. 699-704; Stewart et al, Br Polym J. (1990), 22(4), p. 319-325; and Pispas et al, Polymer (1995) 36(15) p 3005-11. This initiator demonstrates a high initiation efficiency and a fast propagation rate of the growing PL polymer chain.
In accordance with the present invention, it has been found that the tertiary amino or phosphino atoms of these initiators are also quite reactive under onium salt-forming conditions with benzylic halogen present in the backbone polymer chain such that the quaternary ammonium or phosphonium polymer salt is readily formed.
Anionic polymerization to form the graftable PL segments is carried out under well known anionic solution polymerization techniques such as disclosed in U.S. Pat. No. 5,162,445. These polymerizations are normally carried out at low temperatures under vacuum or in an inert atmosphere with rigorously purified and dried reagents and cleaned reactor-ware, since trace moisture or impurities can lead to premature termination of the reaction. The reaction is conducted in an organic solvent which solvates the initiator and monomer reactants. Suitable solvents include benzene, toluene, hexane, tetrahydrofuran, 1,2-dimethyoxyethane and like solvents.
The anionic polymerization is initiated by contacting the organolithium initiator with one or a mixture of anionically polymerizable monomers to form a propagating, living polymer chain. Block copolymer segments may be formed by first contacting the initiator with monomer A to form living pA.Li carbocation segments, followed by contact of these living pA.Li carbocation segments with monomer B to form pApB (AB) block copolymers. Tri (ABA) and other poly block segments may be formed by sequential addition of other monomers following the above described polymerization methodology.
The anionic polymerization may be terminated by the addition to the polymer carbocation of a proton donor, chain transfer agent such as water or a lower alcohol, e.g. isopropyl alcohol. The resulting polymer will comprise a polymer PL having the functional tertiary amino or phosphino group positioned at a single chain end according to the structure R1 R2 Z R3xe2x80x94PL wherein R1, R2, Z and R3 are as described above.
The graft reaction may be accomplished by heating a solution of the halogenated backbone polymer and the functionalized grafting PL polymer to reflux in a suitable organic solvent and under nitrogen for a period of time sufficient to allow the grafting reaction to take place as determined by GPC, generally from about 1 to 24 hours. The graft copolymer may then be recovered from solution by precipitation using an excess of nonsolvent such as an alcohol, e.g. isopropanol, followed by vacuum drying. The graft copolymers may also be produced directly by mixing in the melt state. This is a solventless procedure and is the preferred route for economic reasons. Typical melt mixers include bbanbury mixers, Brabender melt mixers, extruders and other such mixers known in the art.
The degree of grafting (grafting level) of the PL polymer segments is controlled largely as a function of the number of benzylic halogen atoms present in the backbone chain with which the functionalized PL segments may react and the quantity of functionalized PL segments contacted with the halogenated backbone polymer. In general, the graft copolymer may comprise from about 1 to about 70 wt % of the graft PL segments, more preferably from about 10 to 50 wt % of the graft PL segments.
The properties of the graft copolymers and their blend compatibilization efficiency is dependent on the average number of grafts per chain, molecular weight of the graft segments and the polymer structure of the grafts themselves. The latter two parameters are controlled by the anionic polymerization procedure itself. The former parameter can be controlled by the level of benzylic halide groups and the concentration of amino or phosphino functionalized polymers used in the grafting process. The concentration of amino or phosophino terminated polymers may also be controlled by xe2x80x9cdilutingxe2x80x9d the reactive polymer with non-reactive polymer, either added prior to the graft reaction or formed in-situ such as by conducting the anionic polymerization using a mixture of the amine or phosphine functional initiator and a conventional organo lithium initiator, such as sec-butyl lithium, which produces polymer segments non-reactive with the backbone polymer once the anionic polymerization is terminated.
The graft polymer (PL) segments may also comprise a mixture of separately polymerized different polymer segments. For example, reactive polystyrene and reactive polybutadiene polymer chains can be mixed in any proportion prior to contact with the backbone polymer for the grafting reaction to produce both polystyrene and polybutadiene graft segments pendant to the backbone polymer chain.
Graft copolymers comprising a isobutylene polymer backbone chain and a polystyrene graft chain can be used to compatibilize blends comprising isobutylene polymers, including butyl rubber and/or isobutylene/para-methylstyrene rubber, with polystyrene or styrene-containing copolymers. Similarly, graft copolymers containing a isobutylene polymer backbone chain and polybutadiene and/or polyisoprene graft chains will compatibilize blends of isobutylene polymers with polybutadiene and/or polyisoprene.
Also, in the production of pneumatic tires, a number of elastomeric products come in contact with each other in the course of tire assembly. Typically, the adhesion between these materials is poor. This adhesion can be improved using the graft copolymers of this invention either as adhesion layers between butyl and more highly unsaturated elastomer layers or by fabricating one or more tire assembly layers from compositions containing these graft polymers.
Further, these graft copolymers can be used as thermoplastic elastomers if the grafts have a glass transition temperature (Tg) typically above room temperature, e.g. graft copolymers containing styrene, para-methylstyrene, t-butylstyrene, acrylates, methacrylates and the like. Unique elastomeric-elastomeric xe2x80x9cthermoplasticxe2x80x9d elastomers can also be produced when the grafts have a Tg less than room temperature. Especially unique thermoplastic elastomers are formed when the grafts are of a blocky structure with one or more components of the block possessing a Tg greater than room temperature.
Typical tire applications where these materials are useful include sidewall of tires as well as treads, bladders, innnerliners and carcasses. Typical nontire applications include thermoplastic elastomers, mechanical goods, hoses, belting and adhesives, among other uses.
The following examples are illustrative of the invention. The brominated poly (isobutylene-co-4-methylstyrene) polymer (BR-IPMS) used in the examples has a saturated copolymer backbone chain with randomly distributed pendant para-methyl substituted aromatic rings. After polymerization, a solution bromination process is used in which a portion of the para-methyl groups is converted to bromomethyl groups to provide the desired benzyl bromide functionality.
The BR-IPMS elastomer used in the examples has a viscosity average molecular weight of 350,000 g/mole, determined in diisobutylene at 20xc2x0 C., and a Mooney viscosity of 35 (ML 1+8 at 125xc2x0 C.). It contains 1.14 wt % benzyl bromide, as determined by x-ray fluorescence. The benzyl bromide content is measured by FTIR using a master curve calibrated by NMR results.
This BR-IPMS copolymer may be purified for use in the graft reactions by dissolving it in hexane or cyclohexane at a 10 wt % solids content. When fully dissolved, the polymer is precipitated in acetone or methyl ethyl ketone. Redissolve the xe2x80x9cwetxe2x80x9d polymer again in THF at about 10% solids ratio. When fully dissolved, place the slightly cloudy cement solution into suitable sized centrifuge tubes and centrifuge at the highest allowable speed for 40 minutes. When the centrifuge stops, carefully remove the tubes and decant off the precipitate at the bottom of the tube. Precipitate the clear cement with acetone. Place the polymer in a vacuum oven and dry at +70xc2x0 C. until the slightly white precipitate turns into a clear transparent polymer slab. The polymer is now ready for grafting reactions as set forth in Examples 5-7.